In recent years, due to remarkable developments in the art, the heat-sensitive recording method has been widely used in various applications since it has a high reliability, even when implemented in compact recording devices (whose maintenance is free). Various attempts have since been proposed which make it possible to provide multi-colored or full-colored images recorded by the heat-recording.
In general, when the heat-sensitive recording materials are to be used in multi-colored records (as a result of heat recording), the heat-sensitive recording material is applied to the multi-colored records as electron donating dye precursors (for color forming agents) and electron accepting compounds (for color developers), or diazo compounds (for color forming agents) and coupling components (for color developers), which each produces different hues due to the varying levels of the thermal energy applicable thereto (known as color separation).
Such materials are produced by providing on the support the heat-sensitive recording layers in superposition by coating to secure the multilayers (which establish different hues produced by different manners in color formation upon heat-recording). These multi-color heat-sensitive recording materials can suffer from the disadvantage of the color mixing phenomenon or reduced heat-sensitivity due to the fact that the heat-sensitive recording layers thereof become mixed with each other when the layers are provided in superposition by coating, or due to the color forming components or sensitizers diffusing between the respective layers to become miscible after the layers are provided in superposition by coating. To avoid such disadvantages, intermediate layers comprising water-soluble macromolecules as the main constituents are conventionally provided to improve the color separation. Such separation permits one to specify the color forming layer depending on the level of thermal energy applicable thereto. Although the intermediate layers introduce beneficial effects in the color separation and/or in avoiding the diffusion associated therewith, the intermediate layers, disadvantageously, make it complex to produce the multi-color heat-sensitive recording materials.
Thus, the present inventors have already proposed intermediate layers comprising gelatin or gelatin derivatives as the main constituents together with binders contained in the heat-sensitive recording layers to obtain the multi-color heat-sensitive recording materials which are suitable for favorable production. This was done by providing the heat-sensitive recording layers in superposition by coating. As a result, this embodiment satisfactorily avoided the diffusion of the color forming components between the respective layers (e.g., see JP-A-4-35986, wherein the term "JP-A", as used herein, means an "unexamined published Japanese patent application").
However, even such multi-color heat-sensitive recording materials still have the further defect(s) of color-mixing or desensitization due to the diffusion which still exists. This is true because the diffusion of the color forming components between the recording layers cannot completely be avoided (insufficient effect of avoiding the diffusion) when the materials are stored under high temperatures and humid conditions (in view of preservability which means resistance to heat and humidity). When substances known as those typically having an effect in avoiding diffusion are incorporated into such intermediate layers comprising gelatin as the main constituent, such embodiments are prone to additional problems. For example, the resulting coating liquids in these embodiments are of poor stability, which is unsuitable for the favorable production of the multi-color heat-sensitive recording materials. This is true because an interaction between the above-mentioned substances and the gelatin contained therein occurs, which causes the gelatin to become an aggregate or gel.